Project Summary The goal of this project is to determine how ciliopathy-related mutations contribute to ciliary pathophysiology. Ciliopathies are disorders rooted in ciliary dysfunction and exhibit overlapping clinical features, including developmental delay, intellectual disability, polydactyly, retinal dystrophy, and progressive involvement of the kidney and liver. While individually rare, ciliopathies combined affect 1/500 individuals and each of the ~30 distinct ciliopathies is caused by dysfunction of a specific protein network related to the cilium, although the precise cellular mechanisms remain elusive. The primary cilium is an antenna-like projection found on nearly every cell; it extends from the cell body, where it receives and interprets signals, thus allowing cells to respond to their environment. Cilia are partitioned from the cellular cytoplasm by the transition zone that regulates protein trafficking. A dedicated active transport system, intraflagellar transport, moves proteins bound for the cilium across this barrier and works in conjunction with multiple methods for protein retention and selective egress. The proteins involved in this selective protein transport are implicated in a range of ciliopathies, indicating that aberrant ciliary protein content likely contributes to the etiology of these disorders. I will use my novel human cilia isolation protocol and state-of-the-art mass spectrometry approach to assess global ciliary protein composition, defining differences between controls and cells harboring ciliopathy- associated hypomorphic mutations. This work will provide a comprehensive, unbiased catalog of mislocalized proteins in Joubert (K99) and Bardet-Biedl (R00) syndromes, thus providing a rich resource for future work to dissect the protein networks involved in ciliopathies. In a complementary approach, I will determine how ciliopathy-associated mutations affect the dynamics of protein trafficking by endogenously tagging key ciliopathy proteins and following their movement using live-cell microscopy. This work will answer critical questions about the impact of ciliopathy-associated mutations on entry into, retention within, and exit from cilia. This application proposes innovative techniques that are easily extendable to other proteins/ciliopathies, and importantly, investigates protein content and dynamics in the human disease context rather than with null mutations in animal models. Together, this work will shed light on the etiology of ciliopathies and catalyze the development of future therapies.